The present invention related to a system for the utilization of low-grade heat such as solar energy or the waste heat of a power generating plant by utilizing the large variation of the sorption capacity of molecular sieve zeolite. In particular, the system relates to a system which converts small variations in absolute temperature to relatively large variations in gas pressure which, in turn, is utilized to produce mechanical or electrical energy or cooling in refrigeration.
One of the primary difficulties which hinders the utilization of solar energy for heat and cooling purposes is its low energy density (less than 1.5 kilowatt per square meter) of solar energy on earth. The temperature differentials obtained with solar energy collectors are small and even when solar concentrators are used, temperatures above 200.degree.-300.degree. centigrade require sophisticated sun-following techniques. Thus, there is a need for improved efficient energy conversion at small temperature differentials, say between 30.degree.-100.degree. centigrade.
Those skilled in the art understand due to the low temperature differentials obtainable with solar energy, Carnot efficiency of any system using the normal expansion of gases is of necessity quite low. For this reason, most solar energy refrigeration systems have concentrated on the old, well proved absorption refrigeration cycle based on the change of the solubility of a gas in a liquid with temperature. Inasmuch as this process is thermally activated, its dependence on temperature is exponential which permits large changes of gas pressure for small changes in absolute temperature. This process has received new impetus by commercial use of systems other than the ammonia-water used in early gas refrigerators. For example, at Kennedy Airport, New York City, an air conditioning system is provided which utilizes lithium bromide and water as working fluids.
In all refrigeration solid adsorption systems which have operated successfully the heat source, supplied usually by a gas flame or steam, has been about 300.degree. F. Although such systems operate efficiently and with adequate capacity, none has achieved commercial importance. In contrast, solar heat from flat plate collectors rarely exceeds 190.degree. F. and the heat collection efficiency of the collectors is much higher at lower temperatures of 120.degree. to 140.degree. F. Due to lower ranges of temperatures involved and, in particular, the reduced heat available from solar energy as a heat source, concentrated research and development efforts in the last few years, funded both by the Government and private industry, have failed to produce a cooling system which holds commercial promise. For example, modification of a Lithium Bromide system for solar energy has resulted in a drastically reduced and low efficiency, requiring 80.degree. F. water cooled condensers. When the condenser temperatures raises to 120.degree. F., as is necessary for air cooled condensers, a driving temperature at 140.degree. to 160.degree. F., which is reasonably obtainable from flat plate solar collectors, is insufficient for the system to operate.
It is considered that a primary difficulty with applying solar energy to conventional sorption systems is that the physical processes involved are either solution or surface sorption and are exponentially thermally activated in accordance with the simple Arhenius's equation. As a result, the pressure differentials produced by the small temperature differentials are impracticably small and thus useless in most applications.
For specific prior patents which disclose the state-of-the-art, attention is invited to the following:
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